motion trail effect on video in VSE
Rather than doing this in the VSE you can create this effect using a feedback technique similar to How to do this type of trailing effects where the previous frame is held over to include in the current frame. The only real difference is in masking out the current frame so as to overwrite all but the background each time.
The setup of the scene is the same (ie, a camera pointing at the plane) with the following material on the plane :

Here the feedback is handled by the bottom branch - with the Image Texture pointing at the location of the render result, offset by 1 frame to pickup the previous frame. The top branch injects the video (filed infront of a black background) with the Subtract used to darken the image (this was used as a crude way to remove background artifacts since my video background wasn't perfect) with the Less Than used to detect the 'foreground' so that the feedback can be cut out wherever the new image is above a certain threshold. The images are combined and the Emission with a value less than 1 effects the fade out as the trail ages.
This produces the following result :